This invention relates to digital optical communications systems and, in particular, to a laser transmitter with improved optical performance for use in such systems.
In digital optical communications systems, freedom from noise which distorts the detected signal is of prime concern. Where the light is highly coherent, as where a laser operates in a single mode, a persistent problem has been the presence of modal noise. This is caused by destructive interference effects produced in the optical transmission media up to and including the detector end as a result of the light following different paths. The resulting signal will thus often evidence sharp drops in amplitude during a pulse producing low value eye margins and possible errors in the signal. Microwave modulation has been employed in analog optical systems for reducing modal noise (see, Vanderwall et al., "Suppression of Some Artifacts of Modal Noise in Fiber-Optic Systems," Optics Letters, September 1979, Vol. 4, No. 9, pages 295-296. However, a significant noise penalty was incurred. It is desirable to eliminate modal noise without incurring other noise penalties which result in eye margin degradation.
In systems where a multimode laser is utilized modal noise is not a problem. However, such lasers frequently exhibit discontinuities at certain portions of their light output versus current characteristics which result in low frequency noise when the lasers are operated in these regions. Rather than resulting from any interference effects, such noise results from the characteristics of the laser itself such as deep level traps, growth defects in the crystal, etc. The ultimate result is again a degradation in eye margins of the detected signal at the receiver end of the system.
It is therefore a primary object of the invention to increase the performance of lasers in digital optical systems thereby achieving large eye margins for the detected signals.